Electroluminescent device

ABSTRACT

An optical device comprising an anode, a cathode comprising barium, strontium or calcium, and a layer of organic semiconducting material between the anode and the cathode wherein a layer of hole transporting and electron blocking material is located between the anode and the layer of organic semiconducting material.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of copending U.S. patent application Ser. No.10/549,462 filed Aug. 8, 2006, which is the national phase ofPCT/GB2004/001209 filed Mar. 19, 2004, which claims priority to UnitedKingdom Application No. 0306409.4 filed Mar. 20, 2003, the entirerespective disclosures of which are incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates to organic optical devices, in particular organicelectroluminescent and photovoltaic devices.

BACKGROUND OF THE INVENTION

One class of opto-electrical devices is that using an organic materialfor light emission (an organic light emitting device or “OLED”) or asthe active component of a photocell or photodetector (a “photovoltaic”device). The basic structure of these devices is a semiconductingorganic layer sandwiched between a cathode for injecting or acceptingnegative charge carriers (electrons) and an anode for injecting oraccepting positive charge carriers (holes) into the organic layer.

In an organic electroluminescent device, electrons and holes areinjected into the semiconducting organic layer where they combine in togenerate excitons that undergo radiative decay. In WO 90/13148 theorganic light-emissive material is a polymer, namelypoly(p-phenylenevinylene) (“PPV”). Other light emitting polymers knownin the art include polyfluorenes and polyphenylenes. In U.S. Pat. No.4,539,507 the organic light-emissive material is of the class known assmall molecule materials, such as (8-hydroxyquinoline) aluminium(“Alq₃”). WO 99/21935 discloses the class of materials known asdendrimers. In a practical device one of the electrodes is transparent,to allow photons to escape the device.

A organic photovoltaic device has the same construction as an organicelectroluminescent device, however charge is separated rather thancombined as described in, for example, WO 96/16449.

One feature of OLED architecture that has attracted considerableresearch is the selection of the cathode. Factors to be taken intoaccount in selecting a cathode include (a) the workfunction of thecathode relative to the lowest unoccupied molecular orbital (LUMO) ofthe emissive material and (b) the possibility of the cathode degradingthe organic material or vice versa. It will therefore be apparent thatselection of the appropriate cathode for a given material is notstraightforward, and is yet further complicated when the cathode isrequired to be compatible with all three of a red, green and blueelectroluminescent material as per a full color OLED. For example,Synthetic Metals 111-112 (2000), 125-128 discloses a full color displaywherein the cathode is LiF/Ca/Al. The present inventors have found thatthis cathode is particularly efficacious with respect to the blueemissive material but which shows poor performance with respect to greenand, especially, red emitters. For this cathode, the present inventorshave found a particular problem of degradation in green and redefficiency when pixels of these colors are not driven which is believedto be due to migration of lithium into the electroluminescent material.

Some attention has been directed towards cathodes comprising barium. Forexample cathodes comprising elemental barium are disclosed in WO98/57381, Appl. Phys. Lett. 2002, 81(4), 634 and WO 02/84759, andcathodes comprising barium fluoride are disclosed in Appl. Phys. Lett.2001, 79(5), 2001 and in the applicant's co-pending application PCTGB02/03882. However, barium containing cathodes still have drawbacks. Inparticular, the workfunction of barium is relatively high which makesinjection of electrons into the LUMO of the typical blueelectroluminescent material energetically unfavourable. Thus, barium isa relatively inefficient electron injector for blue electroluminescentmaterials at least.

It is therefore an object of the invention to provide an organic opticaldevice wherein the cathode comprises a relatively high workfunctionmetal, but has improved performance, in particular improved lifetime(i.e. the time taken for the brightness of the device to decay to halfits original brightness at a fixed current) and improved efficiency, ascompared to prior art devices with cathodes comprising barium.

GENERAL DESCRIPTION

The present inventors have surprisingly found that the combination of acathode comprising a relatively high workfunction metal such as bariumand a hole transporting/electron blocking layer results in animprovement in both lifetime and efficiency for OLEDs across the rangeof colors.

Barium has a workfunction (wf) of 2.7 eV; strontium has a wf of 2.59 eV;and calcium has a wf of 2.87 eV (source: J. Appl. Phys. 48(11) 1997,4730.)

Accordingly, in a first aspect the invention provides an optical devicecomprising

-   -   an anode    -   a cathode comprising barium, strontium or calcium; and    -   a layer of organic semiconducting material between the anode and        the cathode

wherein a layer of hole transporting and electron blocking material islocated between the anode and the layer of organic semiconductingmaterial.

Preferably, the cathode comprises barium, more preferably elementalbarium. Alternatively, the cathode may comprise a barium compound suchas barium fluoride.

Preferably, the optical device is an electroluminescent device, morepreferably a full color electroluminescent device wherein the layer oforganic semiconducting material comprises red, green and blueelectroluminescent materials.

Preferably, the layer of hole transporting and electron blockingmaterial comprises a triarylamine. More preferably, the triarylamine isprovided as repeat units of a polymer. In this case, the polymer ispreferably a copolymer having one or more arylene co-repeat units.Preferred co-repeat units are selected from optionally substitutedfluorene, spirofluorene, indenofluorene and phenylene, more preferably9,9-disubstituted fluorene-2,7-diyl.

Where the triarylamine is a repeat unit of a polymer, it is preferablyselected from repeat units of formulae 1-6:

wherein X, Y, A, B, C and D are independently selected from H or asubstituent group. More preferably, one or more of X, Y, A, B, C and Dis independently selected from the group consisting of optionallysubstituted, branched or linear alkyl, aryl, perfluoroalkyl, thioalkyl,cyano, alkoxy, heteroaryl, alkylaryl and arylalkyl groups. Mostpreferably, X, Y, A and B are C₁₋₁₀ alkyl. The repeat unit of formula 1is most preferred.

Preferably, the layer of organic semiconducting material is asemiconducting polymer, more preferably a semiconducting copolymer.

Where the organic semiconducting material is a copolymer, preferredrepeat units are selected from optionally substituted fluorene,spirofluorene, indenofluorene and phenylene, more preferably9,9-disubstituted fluorene-2,7-diyl. Preferred 9-substituents areoptionally substituted alkyl and aryl.

Triarylamines of formulae 1-6 are preferred repeat units for thesemiconducting copolymer, most preferably in combination with the repeatunits identified in the preceding paragraph.

One particularly preferred triarylamine repeat unit is an optionallysubstituted repeat unit of formula (I):

wherein each R is independently selected from the group consisting of Hor optionally substituted, branched or linear alkyl, aryl,perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl andarylalkyl groups. Preferably, each R is a C₁₋₁₀ alkyl. Yet morepreferably, each R is butyl, most preferably n-butyl.

Where the organic semiconducting material is a copolymer comprisingtriarylamine repeat units, the present inventors have surprisingly foundthat device performance is improved at lower molar ratios of thetriarylamine repeat unit. Therefore, the molar ratio of triarylamine insuch copolymers is preferably less than or equal to 50%, more preferablyless than or equal to 30% and most preferably 1-10%.

Preferably, a layer of hole injecting material is located between theanode and the layer of hole transporting and electron blocking material.The hole injecting material is preferably a polymer, more preferablypoly(ethylene dioxythiophene).

Preferably, the cathode comprises a first layer in contact with theorganic semiconducting layer comprising elemental barium, strontium orcalcium and a second layer comprising an inert metal. Preferably, theinert metal is silver, aluminium or gold.

In a second aspect, the invention provides a method of forming anoptical device comprising

-   -   providing a substrate comprising an anode;    -   depositing a layer of hole transporting and electron blocking        material onto the anode;    -   depositing a layer of organic semiconducting material over the        layer of hole transporting and electron blocking material; and    -   depositing a cathode comprising barium, strontium or calcium        over the layer of organic semiconducting material

Preferably, a layer of hole injecting material is deposited between theanode and the layer of hole transporting and electron blocking material.

The present inventors have surprisingly found that solution depositionof the soluble hole transporting and electron blocking material producesa layer that is at least partially resistant to dissolution, inparticular resistant to dissolution when the organic semiconductingmaterial is deposited from a solution using a solvent that the holetransporting and electron blocking material is otherwise soluble in.This resistance to dissolution enables the organic semiconductingmaterial to be deposited from solution without completely intermixingwith the hole transporting and electron blocking layer. The presentinventors have found that this resistance to dissolution is obtained (a)regardless of whether a PEDT/PSS layer is used or not and (b) when thehole transporting and electron blocking material is deposited in air orin a nitrogen only environment.

Therefore, the layer of hole transporting and electron blocking materialand the layer of organic semiconducting material are both preferablydeposited from solution. In this case, both the layer of holetransporting and electron blocking material and the layer of organicsemiconducting material are preferably polymers.

The resistance of the hole transporting and electron blocking materialto dissolution may be increased by heat treatment of the layer followingdeposition. Therefore, when the layer of hole transporting and electronblocking material and the layer of organic semiconducting material areboth deposited from solution, it is preferred that the hole transportingand electron blocking layer is subjected to heat treatment prior todeposition of the organic semiconducting material. Preferably, the heattreatment is below the glass transition temperature (Tg) of the holetransporting and electron blocking material. Preferably, the organicsemiconducting material is substantially free of cross-linkable vinyl orethynyl groups.

By “red electroluminescent material” is meant an organic material thatby electroluminescence emits radiation having a wavelength in the rangeof 600-750 nm, preferably 600-700 nm, more preferably 610-650 nm andmost preferably having an emission peak around 650-660 nm.

By “green electroluminescent material” is meant an organic material thatby electroluminescence emits radiation having a wavelength in the rangeof 510-580 nm, preferably 510-570 nm.

By “blue electroluminescent material” is meant an organic material thatby electroluminescence emits radiation having a wavelength in the rangeof 400-500 nm, more preferably 430-500 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows an electroluminescent device according to the invention

FIG. 2 shows a plot of luminance vs. time for a blue electroluminescentdevice

FIG. 3 shows a plot of external quantum efficiency vs. amine content fora series of blue electroluminescent devices

FIG. 4 shows a plot of efficiency vs. bias for a red electroluminescentdevice

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a PLED or photovoltaic device according to theinvention comprises a substrate 1, an anode 2 of indium tin oxide, alayer 3 of organic hole injection material, a layer 4 of holetransporting and electron blocking material, a layer 5 of organicsemiconducting material and a cathode 6.

Optical devices tend to be sensitive to moisture and oxygen.Accordingly, the substrate preferably has good barrier properties forprevention of ingress of moisture and oxygen into the device. Thesubstrate is commonly glass, however alternative substrates may be used,in particular where flexibility of the device is desirable. For example,the substrate may comprise a plastic as in U.S. Pat. No. 6,268,695 whichdiscloses a substrate of alternating plastic and barrier layers or alaminate of thin glass and plastic as disclosed in EP 0949850.

Although not essential, the presence of layer 3 of organic holeinjection material is desirable as it assists hole injection from theanode into the layer or layers of semiconducting polymer. Examples oforganic hole injection materials include poly(ethylene dioxythiophene)(PEDT/PSS) as disclosed in EP 0901176 and EP 0947123, or polyaniline asdisclosed in U.S. Pat. No. 5,723,873 and U.S. Pat. No. 5,798,170.

Cathode 6 comprises a layer containing barium This layer may consistsolely of barium or it may comprise barium and another material, forexample an alloy comprising barium. As an alternative to elementalbarium, cathode 6 may comprise a layer containing a dielectric bariumsalt, in particular barium fluoride as disclosed in Appl. Phys. Lett.2001, 79(5), 2001.

Cathode 6 may also comprise a capping layer of a relatively inertmaterial over the layer containing barium. Suitable inert materialsinclude silver, gold and aluminium.

The device is preferably encapsulated with an encapsulant (not shown) toprevent ingress of moisture and oxygen. Suitable encapsulants include asheet of glass, films having suitable barrier properties such asalternating stacks of polymer and dielectric as disclosed in, forexample, WO 01/81649 or an airtight container as disclosed in, forexample, WO 01/19142.

In a practical device, at least one of the electrodes issemi-transparent in order that light may be absorbed (in the case of aphotoresponsive device) or emitted (in the case of a PLED). Where theanode is transparent, it typically comprises indium tin oxide. Examplesof transparent cathodes are disclosed in, for example, GB 2348316. Anexample of a transparent, barium containing cathode comprises a bilayerof barium and gold.

A typical electroluminescent device comprises an anode having aworkfunction of 4.8 eV. Accordingly, a suitable HOMO level for holetransport by the hole transporting electron blocking material of layer 4for is preferably around 4.8-5.5 eV The LUMO level of the holetransporting electron blocking material of layer 4 for electron blockingfunctionality is preferably shallower (i.e. less positive) than thematerial of layer 5 possessing the deepest (i.e. most positive) LUMOlevel, and is more preferably about 1.6-2.3 eV.

The hole transporting and electron blocking layer may be atriarylamine-containing small molecule, dendrimer, homopolymer orcopolymer. For example, fluorene-triarylamine copolymers are disclosedin WO 99/54385. Another material suitable for use as hole transportingand electron blocking layer 4 is poly(vinyl carbazole) as disclosed in,for example, IEEE Transactions on Electron Devices, 1997, 44(8),1263-1268.

The organic semiconducting material used to form layer 5 may be afluorescent small molecule, dendrimer or polymer. The layer 5 mayalternatively comprise a phosphorescent material doped into a hostmaterial.

Suitable polymers for use as layer 5 include poly(arylene vinylenes)such as poly(p-phenylene vinylenes) and polyarylenes such as:polyfluorenes, particularly 2,7-linked 9,9 dialkyl polyfluorenes or2,7-linked 9,9 diaryl polyfluorenes; polyspirofluorenes, particularly2,7-linked poly-9,9-spirofluorene; polyindenofluorenes, particularly2,7-linked polyindenofluorenes; polyphenylenes, particularly alkyl oralkoxy substituted poly-1,4-phenylene. Such polymers as disclosed in,for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.

Polyarylenes as described above may conveniently be formed by Suzukipolymerisation as disclosed in, for example, WO 00/53656 and Yamamotopolymerisation as disclosed in, for example, “Macromolecules”, 31,1099-1103 (1998). Suzuki polymerisation entails the coupling of halideand boron derivative functional groups; Yamamoto polymerisation entailsthe coupling of halide functional groups. Accordingly, it is preferredthat each monomer is provided with two reactive functional groupswherein each functional group is independently selected from the groupconsisting of (a) boron derivative functional groups selected fromboronic acid groups, boronic ester groups and borane groups and (b)halide functional groups.

The aforementioned polyarylenes may comprise further repeat units suchas hole transporting repeat units 1-6 described above or heteroarylrepeat units. Particularly preferred heteroaryl repeat units includeunits of formulae 7-21:

wherein R₆ and R₇ are the same or different and are each independentlyhydrogen or a substituent group, preferably alkyl, aryl, perfluoroalkyl,thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl or arylalkyl. For easeof manufacture, R₆ and R₇ are preferably the same. More preferably, theyare the same and are each a phenyl group.

Further suitable arylene repeat units are known in this art, for exampleas disclosed in WO 00/55927 and WO 00/46321, the contents of which areincorporated herein by reference.

A single polymer or a plurality of polymers may be deposited fromsolution to form layer 5. Suitable solvents for polyarylenes, inparticular polyfluorenes, include mono- or poly-alkylbenzenes such astoluene and xylene. Where a plurality of polymers are deposited, theymay comprise a blend of at least two of a hole transporting polymer, anelectron transporting polymer and, where the device is a PLED, anemissive polymer as disclosed in WO 99/48160. Alternatively, layer 5 maybe formed from a single polymer that comprises regions selected from twoor more of hole transporting regions, electron transporting regions andemissive regions as disclosed in, for example, WO 00/55927 and U.S. Pat.No. 6,353,083. Each of the functions of hole transport, electrontransport and emission may be provided by separate polymers or separateregions of a single polymer. Alternatively, more than one function maybe performed by a single region or polymer. In particular, a singlepolymer or region may be capable of both charge transport and emission.Each region may comprise a single repeat unit, e.g. a triarylaminerepeat unit may be a hole transporting region. Alternatively, eachregion may be a chain of repeat units, such as a chain of polyfluoreneunits as an electron transporting region. The different regions withinsuch a polymer may be provided along the polymer backbone, as per U.S.Pat. No. 6,353,083, or as groups pendant from the polymer backbone asper WO 01/62869. In view of the hole transporting and electron blockingproperties of layer 4, such properties may optionally be excluded fromthe polymer or polymers comprising layer 5.

Layers 4 and 5 may be formed by any process for forming multiple layersin organic optical devices. It is desirable to minimise intermixing oflayers 4 and 5; in the case of small molecules, this is straightforwardbecause each layer is typically deposited by evaporation under vacuum.In contrast, dendrimers and polymers are typically deposited fromsolution and so it is desirable to employ a technique to preventintermixing when layer 5 is deposited from solution onto a solutiondeposited layer 4. One technique for preventing intermixing of multiplesolution deposited layers is the heat treatment step described in thesecond aspect of the invention, however other techniques are known. Forexample IEEE Transactions on Electron Devices, 1997, 44(8), 1263-1268,describes solution deposition of two materials that require differentsolvents, i.e. the solvent used to deposit the second layer of amultilayer does not dissolve the underlying layer. Another techniquedisclosed in, for example, WO 94/03030 is to use an insoluble PPV as thefirst layer by depositing a soluble “precursor” polymer and then heatingit to form the insoluble PPV onto which a further layer may be depositedfrom solution. Yet another technique disclosed in, for example, U.S.Pat. No. 6,107,452, is to deposit from solution a layer of materialcomprising cross-linkable groups and then treating the layer undercross-linking conditions, e.g. heat or UV treatment, to form a layer ofinsoluble material onto which a further layer may be deposited fromsolution. The material may be a monomer or a polymer comprisingcross-linkable groups.

The optical device prepared according to the method of the invention ispreferably a PLED when the first and second electrodes inject chargecarriers. In this case, layer 5 is a light emitting layer.

The optical device is preferably a photovoltaic device or photodetectorwhen the first and second electrodes accept charge carriers. In thiscase, layer 5 comprises a material capable of electron transport.

Electroluminescent devices may be monochrome, multicolor or full color.Processes for production of monochrome displays include spin coating anddip-coating. Processes for production of full color displays includeinkjet printing as described in, for example, EP 0880303 and laserinduced thermal imaging as disclosed in, for example, EP 1003354.

EXAMPLES General Procedure

The invention is exemplified here using the polymer “F8-TFB”,illustrated below and disclosed in WO 99/54385, as the hole transportingand electron blocking material.

The general procedure follows the steps outlined below:

1) Depositing PEDT/PSS, available from Bayer® as Baytron P® onto indiumtin oxide supported on a glass substrate (available from Applied Films,Colorado, USA) by spin coating.

2) Depositing F8-TFB is by spin coating from xylene solution having aconcentration of 2% w/v.

3) Optionally heating the device at 180° C. for 1 hour.

4) Optionally spin-rinsing the substrate in xylene to remove anyremaining soluble F8-TFB.

5) Depositing a layer of semiconducting polymer by spin-coating fromxylene solution to a thickness of around 750 Å.

6) Depositing by evaporation over the semiconducting polymer a cathodeconsisting of a first layer of barium metal in contact with thesemiconducting polymer and a capping layer of aluminium over the layerof barium metal.

7) Encapsulation of the device using an airtight metal container asdescribed in EP 0776147.

Example 1 Blue Electroluminescent Device

The general procedure above was followed, including optional step 3 butexcluding optional step 4. The organic semiconducting polymer used was ablue electroluminescent copolymer of the following repeat units: 65 mol% 9,9-di-n-octylfluorene-2,7-diyl, 30 mol %9,9-diphenylfluorene-2,7-diyl, and 5 mol % of the repeat unit “PFB”(represented below). The polymer was prepared by Suzuki polymerisationaccording to the method described in WO 00/53656.

For the purpose of comparison, an identical device was prepared exceptthat the layer of F8-TFB was not deposited (i.e. exclusion of steps2-4).

Despite the absence of cross-linking groups, the layer of F8-TFB isresistant to dissolution in xylene under the conditions typicallyemployed for polymer deposition. Although dissolution of the F8-TFBlayer in a solvent may be possible under forcing conditions, it will beappreciated that this layer is sufficiently resistant to dissolution toenable the formation of a plurality of electroactive organic layers.Without wishing to be bound by any theory, a possible mechanism for lossof solubility is an adhesion to the surface that the firstsemiconducting polymer is deposited onto.

As can be seen from FIG. 2, the inclusion of an “interlayer” of F8-TFBresults in a very significant increase in lifetime.

Without wishing to be bound by any theory, the improvement in lifetimemay be due to the F8-TFB layer preventing electrons from flowing intothe PEDT:PSS and/or anode (i.e. hole injection) layers.

Example 2 Blue Electroluminescent Device with Variation of Amine Content

A series of devices was prepared by following the procedure of example 1except that the molar ratio of PFB repeat units within the polymer wasvaried from 2.5% up to 20% (the ratio of diphenylfluorene was keptconstant at 30%; the change in PFB ratio was accommodated by increasingor decreasing the ratio of diphenylfluorene).

For the purpose of comparison, an identical series of devices wasprepared except that the layer of F8-TFB was not deposited (i.e.exclusion of steps 2-4).

As can be seen from FIG. 3, the external quantum efficiency (EQE) of thedevice according to the invention is at least similar to, and at low PFBcontent significantly greater than, comparative devices wherein thelayer of F8-TFB is absent.

Without wishing to be bound by any theory, it is believed that low aminecontent leads to superior device performance because barium is believedto be a relatively poor electron injector. Therefore, charge is balancedwhen the quantity of PFB repeat units, which are capable of transportingholes, is reduced. Similarly, selection of a good electron injector (forexample dielectric materials, in particular fluorides such as lithiumfluoride) may show optimal performance at higher molar ratios of amine.

Example 3 Red Electroluminescent Device

The procedure of Example 1 was followed, except that the organicsemiconducting material used was a red electroluminescent polymer of thefollowing repeat units: 50 mol % 9,9-di-n-octylfluorene-2,7-diyl, 17 mol% “TFB” repeat units (illustrated below), 30 mol %1,3,2-benzothiadiazole-4,7-diyl, and 3 mol %4,7-bis(2-thiophen-5-yl)-1,3,2-benzothiadiazole. Materials of this typeare disclosed in WO 00/46321 and WO 00/55927, the contents of which areincorporated herein by reference.

FIG. 4 shows that efficiency of the device comprising a layer of F8-TFBis higher than the comparative device wherein this layer is absent. Theimprovement is most significant at low voltage.

Furthermore, lifetime of the red electroluminescent device was at leastdoubled as compared to the comparative device.

Example 4 Green Electroluminescent Device

Devices were prepared in accordance with the process of Example 1,except that the organic semiconducting material used was a greenelectroluminescent polymer as disclosed in, for example, WO 00/55927 andWO 00/46321.

As for the red and blue electroluminescent devices, substantialimprovements were observed for both lifetime and efficiency as comparedto a device not comprising the layer of F8-TFB.

It will be apparent that devices according to the invention showimprovement for OLEDs across a wide range of colors and as such areparticularly suitable full color displays, i.e. those comprising red,green and blue electroluminescent materials.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention as set forth in the followingclaims.

1. A method of forming an optical device comprising providing asubstrate comprising an anode; depositing a layer of hole transportingand electron blocking material onto the anode; depositing a layer oforganic semiconducting material over the layer of hole transporting andelectron blocking material; and depositing a cathode comprising barium,strontium or calcium over the layer of organic semiconducting material.2. A method according to claim 1 comprising depositing a layer of holeinjecting material between the anode and the layer of hole transportingand electron blocking material.
 3. A method according to claim 1comprising depositing both the layer of hole transporting and electronblocking material and the layer of organic semiconducting material fromsolution.
 4. A method according to claim 3 wherein both the layer ofhole transporting and electron blocking material and the layer oforganic semiconducting material are polymers.
 5. A method according toclaim 3 comprising subjecting the hole transporting and electronblocking layer to heat treatment prior to deposition of the organicsemiconducting material.
 6. A method according to claim 5 wherein theheat treatment is below the glass transition temperature of the holetransporting and electron blocking material.
 7. A method according toclaim 3 wherein the organic semiconducting material is substantiallyfree of cross-linkable vinyl or ethynyl groups.